Our studies in this area have focused on two specific effects of Hcy on immune cells and their function, namely cellular apoptosis and cytokine expression. Hcy has been shown to induce apoptosis in a number of cell lines and cell type including immune cells. However, very little information is available on how the various forms of Hcy influence T-cell apoptosis or the specific mechanisms by which cell death is triggered. Our studies have revealed that treatment of resting and activated human and murine T cells with L-Hcy results in a dose-dependent increase in apoptotic cell death. The L or D,L forms of Hcy was more potent than Hcy thiolactone in this respect, while S-adenosyl Hcy (SAH) was found to be significantly less active. We also found that the pro-apoptotic effects of Hcy were abrogated with the addition of serum, pan- and specific-caspase inhibitors and PARP inhibitors to the cultures. These results suggest that L-Hcy, like other apoptotic stressors, leads to the activation of the caspase cascade and eventually to the cleavage of the key cellular proteins facilitating the release of mitochondrial cytochrome c, DNA fragmentation, and eventually leading to the typical morphological changes observed in cells undergoing apoptosis. Moreover, we have also found that Hcy appears to potentiate cellular death induced by a number of other established apoptotic signals including activation-induced cell death (AICD), heat shock, and Fas ligand- and HIV-mediated T cell death. In vivo infusion of L-Hcy but not SAH induced a dramatic reduction in thymocyte numbers (in particular CD4+CD8+ T cells), thymic weights and a dramatic change in thymic morphology. While these findings are provocative, a specific role for Hcy in age- or disease-associated thymic atrophy remains to be defined. In addition, we have found that activation of purified human or murine T cells in the presence of L-Hcy or thiolactone but not SAH results in a significant increase in the expression of several Th1 and Th17 but not Th2 cytokines. These effects were also observed upon in vivo infusion of Hcy into normal and TCR transgenic mice. In addition, we have found that immune cells actually produce the various forms of Hcy upon activation and that these various Hcy appear to play an endogenous regulatory role in T cell activation and function. Additional work has been initiated using mass spectroscopy to identify and quantitate various forms of Hcy in culture supernatants and serum/plasma. We believe that developing our ability to screen these Hcy mediators may elucidate potential mechanisms of immune activation associated with cardiovascular and inflammatory disease states. Finally, we have recently found that Hcy treatment of CD4+ T cells results in significant IL-17 expression and appears to induce Th17 cells. The precise mechanism involved in the generation of these cytokines is currently under investigation Overall, Hcy appears to exert a number of differential effects on immune cells, which may alter immune function in the circulation and tissue microenvironment with age and inflammatory and/or autoimmune disease pathology. A greater understanding of the potential modulatory effects of Hcy and its metabolites on immune function may result in the development of potential therapeutic strategies to control and optimize immune responses with age, AIDS and in various age-associated disease states.